Despite considerable progress in cochlear implant (CI) technology over the past three decades, speech perception via a CI remains considerably poorer than with normal hearing (NH), particularly in noisy backgrounds. Similar difficulties, although often to a lesser extent, are experienced by hearing-impaired (HI) listeners, whose hearing loss is not severe enough to warrant a CI, even after hearing-aid fitting. The long-term goal of this research is to improve auditory and speech perception via CIs and hearing aids, through a greater understanding of the basic mechanisms that contribute to, and limit, the perception of speech in challenging acoustic conditions. This goal is addressed through three specific aims. The first aim is to combine behavioral and non-invasive neural measures to better understand auditory context effects in NH, HI, and CI populations. Context effects are a crucial part of our perceptual experience, and help to maintain perceptual constancy ? the ability to recognize objects, voices, and words, in the face of different room acoustics, talker properties, and varying background noise. Little is known about how context effects are altered by aging, hearing loss, or CIs. The second aim is to study acoustic and linguistic context effects in speech, and to understand how age and hearing loss interact with this important class of context effects. The third aim is to understand the peripheral and more central contributions to individual differences in the outcomes of CI users. It is often assumed that peripheral and implant-related factors can explain a significant proportion of the variance in CI outcomes. This aim will provide a direct test of the assumption by comparing the variability among CI users with the estimated population variance among younger and older NH listeners under degraded listening conditions in both psychoacoustic and speech-based measures of performance, using larger samples than have been tested in the past. Overall, the results will provide new insights into the spectro-temporal processing of auditory and speech stimuli by NH, HI, and CI populations that will help in the treatment and rehabilitation strategies for people with hearing loss. Treatments include the incorporation of missing context effects that assist perception in varying acoustic conditions via signal processing in hearing aids and CIs, and rehabilitation may include training strategies that accelerate the ability of HI and CI patients to utilize linguistic context cues in everyday conversational environments.